Coupling device



Feb. 6, 1945. C, B WATTS, 2,368,694

COUPLING DEVICE y Filed Aug. l5, 1942 Z//A/Par) lNvlaN-rcznla CHESTER B. WATTSl JR.

hema tu.- s, 194s mittal UNITED Vsfra'riz'sf PATENT oFFicE Chester B. Watts, Jr., East Orange, N. J., assignor to Federal Telephone and-Radio Corporation, a corporation of Delaware Application August 15, 1942', Serial No. 455,003

1o claims. (ci. 11s-44) This invention relates to coupling devices and one of the objects of the invention is to provide improved means for coupling two circuits together so that unbalanced currents and/or, voltages occurring in one of the two circuits are prevented from spreading to the other thereof.

It isv generally regarded that in a two-wire radio frequency transmission line only balanced currents and voltages are present and apparatus associated with the circuit is accordingly designed with this in mind. A However, it is not unusual forA various reasons to have unbalanced components of current and voltage crop up in the circuit. Unless particular pains are taken to cope with unbalanced current and voltagecomponents, they are likely to spread throughout the circuit without any special relation t0 the balanced components thereby causing unexpected and undesirable eiects. Y

The term balanced" as used herein is given its usual meaning that for two-wire transmission line circuits the respective voltages of the two Awires with respect to ground are equal in magnitude and opposite inphase, and that the currents thereof are likewise. The term unbalanced also is used in its usual sense. that the respective voltages` and currents of the two wires are equal in magnitude and have the same phase. Any arfigether by a device comprising aloop conductor having two legs co-axially shielding the major Vportion of the loop. Employed with this prior device is a separate tunable circuit inductively coupled `with the shield. 'This coupling arrangement enects a` coupling between a circuit connected on the one handto the conductor loop and another circuit connected on the other hand to the tunable circuit," whereby unbalanced currents and voltages present in one of the circuits are prevented froin being transmitted from one to the other of the coupled circuits.

be tuned to resonance at the frequency of the input 'current of one of the circuits by either a condenser connected between the legs thereof or by a short circuiting bar movable along the lengths of the legs. I have also discovered that not only a real impedance at the terminals of the conductor loop may be obtained by adjustment of the condenser or the short circuiting bar, as the case may be, but also by changing the position of the points at which a circuit, such as a resistor load,

' is connected to the shield, the magnitude of this real impedance may be varied. In general, the` value of the real impedance at the input decreasesv as the load resistor is moved away from the ex#- .posed portion of the loop conductor. I have found that a very well balanced load effect can be produced by the coupling feature of my invention independent of whether or not the resistor orresistors used are balanced to ground.v Thus, even where two very unequal resistors are connected, one to one leg and the other to the other leg of the shield. the impedance at the input of the conductor will be balanced. y

It is another object of this invention, therefore, to provide simplified coupling means for coupling Y two circuits together wherein the shield portion of the coupling means is tunable to obtain resonance at the frequency of the input voltage.

Another object pt tliis invention is to provide means for coupling together a dual wire trans mission line and a co-axial line.

The above objects and others ancillary thereto will become more apparent upon consideration of the following detailed description tobe read in connection with the accompanying drawing, in which, l

Fig. l is a cross-sectional view of a coupling ,device in accordance with this invention;

wire transmissionline to a co-'axial transmission line.

Referring to Fig. 1 of the drawing. a coupling device in accordance with this invention is shown Inaccordance withmy invention, l. have dis. Y

`Acovered that' for most such coupling arrange-A ments, the separate tunable circuit can be elimilired, eonnectedtotheshield. I'have Y usted and nature circuit formerly associated withtheseparatetunablecircuitcanbe,ifde-' to comprise a loop conductor 20 having terminals 2| and 22 adapted to be .connected to one ofthev .circuits CI to be coupled.- VA shield 25 is provided for the loop conductor 20. This shield comprises a shielding plate 26 having mounted thereon a pair of substantially L'shaped tubular members 21 and 28. These tubularfmembers Idisposed at right angles to the platen and form tovwavelength long.

gether a split loop, that is, the ends 30 andj 3| of the tubular members are disposed toward each other gwith' a gap -32 therebetween. The main portion of the loop conductor 20 is disposed in coaxial shielded relation within the tubular members with a small part thereof exposed at the gap 32. The lengths of the tubular members 21 and 28 are equal and the gap 32 is smal1 compared to the length of -the tubular members.

This shielding arrangement of the loop conconnected to the legs 21 and 23 of the shield.

Since the conductor and the tubular legs prise a resistor Ri. or some other load, is directly 21 and 28 are shown identical in all of the figures of the drawing I have used the same reference characters for these parts throughout the drawing. Y

In the case of a load Rz. (Fig. 1),-the input impedance Zmput) may be easily calculated, as

suming that the legs 21 and 2B are a quarter The loop conductor 2t is treated as a single wire co-axial line havingsurge impedanceZo, carrying current I at the input end'whose`component parts are I+. which comprises the forward wave progressing to the right and I, which is the reflected wave progressing to the left. Thus, I=I1-iI-.'V At the right-hand end of the device, one quarter wavelength away from the left-hand end, the current may be expressed as dual transmission line having surge impedance Z1 with current l at the left-handexd thereof.

The voltage across the shield at the left-hand end is equal to zero and may be expressed by the following equation:

'Ihe voltage E1 across the gap 32 is equal and opposite to the voltage E: at the right-hand end of each leg between the inside of the shield and the conductor 20. This relationship may be .ex

pressed by the following equation:

z1 n+in -zu-m-iI-i k2) Since the voltage Ei across the gap32 is equal to' the voltage across the load`resistor Rz. an

equation -may be `written vas follows:

` z1(-i11+i11- =ILRL (a) 'rneveurret sowing on the shield at the righthand end thereof is equal to the current flowing into the load resistance plus the current flowing A in the conductor'20 at the gap 22 and this relation may beexpressedy as follows:

l The foregoing equations may be simplified -a's indicated in the following live equations:

Solving Equation 6 by using the Equation 5 which in effects states that I1+=I1 it follows that:

Substituting for ik. in Equation 12 the value given in Equation 9, it follows that:

it will be clear that the input impedance is independent 'of the surge impedancev Z1 of *the Shield itself at least as long as the shield istuned to one quarter wavelength, but that it varies with the square of the surge impedance Zo of .the conductor 2li and inversely with the load resistanceRr.. Thus, the impedance at the terminals 2l, 22 of the loop conductor 2li will be real and y balanced.

Should a shield not exactly one quarter wavelength Along be used, it may be eiectively made resonant by a condenser 36 (Fig. 2) or by a movable lsnorting bar 38 (Fig. 3). Thus tuned, the

impedance effect is substantially the saine'as calculated above. If the shield'is not resonant, then the input impedance contains reactance in addition to the resistance.

Referring to Fig. 2 of the drawing, 'a form ei coupling device in accordance with this invention isfshownprovided with a variable condenser 86 connected across the legs 21 and 28 of the shield whereby the shield may be tunedl to resonance at' the frequency of' the. input current.

The condenser 36 may be connected between the legs 21 and 28 at any points therealong as desired. It will be observed, however, that the samecondenser will have a sharper tuning effect when" connected close to the gap 32, and that the tuning effect is decreased as the connections to the condenser are .moved along the length of the legs 21 and 28 toward thefleft-hand end thereof. The circuit having the load R1. may be con.-

nected between 4the legs 21 and 2l as shown in Fig. 1 or the connections thereof may be arranged with movable contact means for'adiustment along the length` of the legs as indicated in Fig. 2. As'tlie load resistor Rx. is moved away lfrom the gap 32, the value of the real impedance I at the input terminals of the conductor decreases. T hus, the value of the real impedance at the input terminals 2|, 22 may be varied by moving the connecting points of the load circuit. InFig. 3 of the drawing, another form of the invention is shown wherein the shield is tuned by a shorting bar 38 movable along the lengthl of the legs 21 and 28. The load Rain this form may be connectedto the legs 21 and 28 as shown in Fig. 3 or it may be connected thereto in the manner of the resistorsV shown in Figs. l, 2

andfi. The movement of the shorting bar 38 varies the tuning of the shield. in the same manwill be balanced.

The tuned coupling device of this invention may be used to couple a source of energy to the loop conductor 20 for transmission by a line 5I to a load 50 such as an antenna. This application of the coupling means of this invention is shown in Fig. 5. An amplifier plate circuit 52 is shown provided-with a tuned circuit 53 disposed in the vicinity of the coupling device. The shield of the coupling device may be provided with a suitable condenser 56 similar to the form shown as illustrative of the invention only and not 'as restricting the appended claims. What-I claim is:

, 1. A device for coupling two circuits together to prevent unbalanced voltages and currents occurring in onefrom spreading to the other of two circuits comprising a continuance loop con- A ductor connected to one of said circuits, al shield for said conductor,V said shield having two hollow legs the free ends of which extend toward oneV another to provide a gap therebetween, said loop cnductor being co-axially mounted withinsaid l legs whereby it is shielded along the lengths of the legs and exposed at said gap, at least one of Asaid legs being connected to the`v other of the two circuits, and said legs being of an electrical' length in the `order of substantially one quarter wavelength of the current to be transmitted from one to the other ot said circuits.

'2. A device for coupling two circuits together to prevent unbalanced voltages and currents occurring inl one from spreading to the other of two circuits comprising a continuous loop conductor. connectable to one of said circuits, a shield for said conductor adapted to be coupled to the other of said circuits, said shield having two holin Fig. 4 by which the shield can be tuned `to the frequency of the current carried by the tuned circuit 53. The effective coupling of the ampliner plate circuit 52 to the line 5I may be varied by controlling the tuning of the condenser 56. Thus, energy may be transmitted to the antenna 50 as desired, and at the same time the transmission of unbalanced currents and voltages between the two circuits may be blocked. Insteadl o f the coupling arrangement shown, the circuit 53 may be broken and the two sides thereof directly connected to the shield; that is, one side of the circuitv 53 may then be connected to the l leg 21 and the other side to the -leg 28.

Another application of the coupling device of this invention is shown in Fig. 6 wherein the device is used to provide a coupling between a twowire transmission line.60 and a co-axial transmission line 62; The shield comprises a box structure 64 having a partition 65 upon which is mounted a shield in accordance with the form shownin Fig. 4, for example, having legs 21 and 28 anda tunable condenser 4S. One end ofthe box structure v64 isA providedwith an opening 66 through which the transmission line 60 is adapted to be connected to the terminals of the loop conductor 20. The opposite end of the boxstructure 64 is provided with an opening B1 to receive for connection to the end wall of the structure, the outer' conductor 68 of thel co-axial transmission line B2. The inner conductor 69 is extendedVy into the box structure andvconnected to one of. the legs oi the shield. l

. While I have shown and described several ein-.- bodiments of the invention, I recognize that .many additional embodiments and variations are possible without departing'irom Vthe invention. It is to be understood, thereiore, that the forms low legs the free ends of which extend toward one another to provide a gap therebetween, said loop conductor being co-axiallyniounted within said legs whereby it is shielded along the lengths of the legs'and exposed at said gap, said legs being of an electrical length in the order of substantially one quarter wavelength of the current to be transmitted from one t0 the otherY of said circuits, and means to tune said shield to resonance at the frequency of said transmitted current.

3. 'I'he coupling device as defined in claim 2 wherein the tuning means comprises a condenser 40 connected between the legs of the shield.

. 4. The coupling device as dennen in claim 2 wherein the tuning means comprises a snorting bar disposed between the legs of the shield and f adjustable along the lengths thereof.

5. The coupling device as dened in claim 2 wherein the circuit coupled to the shield is connected to both of the legs thereof.

6. The coupling device as dened in claim 2 wherein the circuit coupled to the shield comprises means conductively connecting the circuit 8. Apparatus for coupling to a balanced duall transmission line comprising a co-axialline, a

herein shown and described are to be regarded u housing having a partition therein, a pair of L- shaped tubular legs substantially completely enclosed within said housing and mounted on said partition with the long portions thereof disposed in parallel relation and the short portions thereof disposed in opposed relation with the ends thereoi' spaced apart, a loop conductor disposed coaxially with respect to the tubular legs and spanning the space between the ends thereof, said loop conductor being connectable to the dual line, the outer conductor `or the co-axial line being connectable to saidhousing and the inner con..

, dum:- of uw 0.1mm une being connectableto one of said legs, whereby unbalanced voltages and wherein a condenser is connected 'between the 1o. The coupimg device as aenned m claim s currents occurring in one of the said lines will legs to tune them to resonance at the frequency be prevented from spreading to the other line. of the current to be transmitted from one to the 9. The coupling l-device as dened in claim 8 5 other ot said lines.

wherein means are provided for tuning the pair of l less to resonance at the frequency of the current .transmitted from one tothe other otsaid lines. 

